A principle of maximally redundant trees (Maximally Redundant Trees, MRTs) is: on a network on which hop-by-hop forwarding is performed based on a shortest path first (Shortest Path First, SPF) topology, as shown in FIG. 1A, it is assumed that a network includes nodes A, B, C, D, E, F, G, H, I, J, R, and so on; for a common root node (for example, node H in FIG. 1A), two non-intersecting redundant trees (for example, a topology in FIG. 1B and a topology in FIG. 1C) may be calculated, and the redundant tree in FIG. 1B and the redundant tree in FIG. 1C both include all the nodes A, B, C, D, E, F, G, H, I, J and R, and have the same Root node H, but directional links do not overlap.
With a network formed by nodes A, B, C and D as an example, three topologies may be calculated by using an MRT:
Default topology (referring to FIG. 2A): node A→node B→node C and node A→node D→node C;
First topology (referring to FIG. 2B): node A→node B→node C→node D→node A; and
Second topology (referring to FIG. 2C): node A→node D→node C→node B→node A.
A route in the default topology may be protected by using a backup route in the first topology or the second topology. As shown in FIG. 2D, a route in the default topology is protected by the second topology that is shown in FIG. 2C and calculated by using an MRT. When a directional link from node B to node C encounters a fault, traffic is switched to a directional link from node B to node A in the second topology, and arrives at the destination node C via node 1D.
MRT FRR (MRT Fast ReRoute, MRT fast reroute) includes two forwarding mechanisms. One is forwarding by using Label Distribution Protocol (Label Distribution Protocol, LDP) multi-topology, that is, labels are distributed to routes in the first topology and the second topology in a same way and forwarding is implemented in different topologies by using Multiprotocol Label Switching (Multi-Protocol Label Switching, MPLS) labels; and the other is forwarding through Internet Protocol (Internet Protocol, IP) tunnels, where the MPLS technology is not used in this method and a destination address is reached through an IP tunnel in the first topology or the second topology.
The inventor finds during research that the forwarding mechanisms used in MRT FRR have the following problems:
For forwarding through IP tunnels, multi-topology addresses need to be planned in advance, and a large number of IP tunnels are established and maintained on a network, so that management and maintenance of the network become complex.
Challenges also exist when MRT FRR forwarding is supported by using LDP MT (LDP Multi-Topology, LDP multi-topology): 1. For a same prefix, LDP MT-based MRT FRR requires that labels be allocated separately to three topologies, and a large number of labels need to be used. For an IP-only network, additional LDP MT needs to be deployed to support MRT FRR. The number of route prefixes of an Internet backbone network can reach 400,000 to 500,000. If only LDP MT is used, the number of labels in use may exceed 220, that is, existing label space (a label is represented by using 20 bits) may be incapable of meeting an application requirement. 2. For an IP-only network, for ease of operation and maintenance of a network, an operator does not want to introduce a new MPLS protocol (such as the LDP), so that actual deployment of LDP MT-based MRT FRR is also difficult.